各位大侠，目前遇到一难题，题目如下：创建一个类，然后继承一个类，然后由这两个类生成一个对象。
请高手给出程序，不甚感激！

class 类A{
.......
};
class 类B 继承方式:类A
{
.......
}

main()
{
 A 对象a;
 B 对象b;
}

#include <iostream>
using namespace std;
class Shape
{

protected :
    double length,width,area ;
public:
    Shape(double Leng,double Wid,double Area):length(Leng),width(Wid),area(Area)
    {}
     virtual double GetArea() const { return area;}
};
class Rectangle : public Shape
{
public:
    Rectangle(double Leng=0,double Wid=0,double Area=0):Shape(Leng,Wid,area)
    { area=length*width;}
};
class Triangle : public Shape
{
public:
    Triangle(double Leng=0,double Wid=0,double Area=0):Shape(Leng,Wid,area)
    {area=length*width/2;}
};
int main()
{
    Rectangle r(10,20);
    cout<<r.GetArea()<<endl;
    Triangle p(90,45);
    cout<<p.GetArea()<<endl;
}

简单的例子

父类：
class A
{
...........
};
子类：
class B public(private或protected):A
{
....................
}
int main()
{
    A a;
    B b;
    .................
    return 0;
}